A lithium-ion secondary battery includes therein a positive electrode and a negative electrode which are separated by a battery separator. In production of secondary batteries, a separator roll is used. The separator roll is obtained by rolling up a film separator. Because the secondary batteries vary in shape and size, a wide separator original sheet, having been produced is slit in accordance with a width of a secondary battery. As a result, the wide original separator sheet is processed into a plurality of separator rolls. Then, the separator rolls each are rewound so as to be sandwiched between a film positive electrode and a film negative electrode, so that these electrodes and the separator are laminated.
Patent Literature 1 discloses a technique according to which a thickness of a separator is controlled by measuring an outer diameter of a separator roll.